Semiconductor Oxidation Apparatus and Method of Producing Semiconductor Element

ABSTRACT

A semiconductor oxidation apparatus is provided with a sealable oxidation chamber defined by walls, a base provided within the oxidation chamber and configured to support a semiconductor sample, a supply part configured to supply water vapor into the oxidation chamber to oxidize a specific portion of the semiconductor sample, a monitoring window provided in one of the walls of the oxidation chamber and disposed at a position capable of confronting the semiconductor sample supported on the base, a monitoring part provided outside the oxidation chamber and capable of confronting the semiconductor sample supported on the base via the monitoring window, and an adjusting part configured to adjust a distance between the base and the monitoring part.

TECHNICAL FIELD

The present invention generally relates to semiconductor oxidationapparatuses and methods of producing semiconductor element, and moreparticularly to a semiconductor oxidation apparatus and a method ofproducing semiconductor element, which oxidize a semiconductor elementincluding aluminum (Al) and arsenide (As) from an outer peripheral edgeportion towards a central portion, and are particularly suited forproducing an oxidized constricting type surface-emitting laser in whichsizes of a current constricting part and a current injecting part areappropriately adjusted.

BACKGROUND ART

There are semiconductor lasers having a current constricting structurein order to improve the current flow efficiency. A vertical-cavitysurface-emitting laser (VCSEL) is an example of such a semiconductorlaser. The VCSEL emits light in a direction perpendicular to asubstrate, and compared to the so-called edge-emitting semiconductorlasers, the VCSEL has low cost, low power consumption, small size, highperformance and is suited for application to two-dimensional devices.For this reason, much attention is recently drawn to the VCSEL.

An AlAs selective oxidation constricting structure is popularly used forthe current constricting structure of the VCSEL, as may be seen from aU.S. Pat. No. 5,493,577, for example. This current constrictingstructure of the VCSEL is formed by placing a semiconductor substrate ora semiconductor sample including a circular or rectangular base shapedmesa structure within a high-temperature steam atmosphere, and oxidizinga p-AlAs selective oxidized layer included in the mesa structure from anouter peripheral edge that is exposed at a side surface of the mesastructure towards a central portion in a state leaving the centralportion non-oxidized so as to form an Al_(x)O_(y) current constrictingpart (oxidized region). In the VCSEL having the Al_(x)O_(y) currentconstricting part that is formed in the above described manner, theindex of refraction of the Al_(x)O_(y) current constricting part isapproximately 1.6, and is low compared to indexes of refraction of othersemiconductor layers. For this reason, a difference is introducedbetween the indexes of refraction in a lateral direction within aresonant structure, and it is possible to trap the light at the centerof the mesa structure. Thus, desirable characteristics are obtained inthat the current constricting efficiency of the semiconductor element isgood and the threshold current is low.

In order to obtain the single basic lateral mode oscillation in theVCSEL, it is necessary to make the size (for example, the diameter) ofthe current constricting part must be made small and the diffractionloss with respect to the high order mode must be made large. Moreparticularly, the size of one side or the diameter of the currentconstricting part must be made narrow to approximately 3 toapproximately 4 times the oscillation wavelength. For example, if theoscillation wavelength is 0.85 μm or 1.3 μm, the size of one side or thediameter of the current constricting part must be approximately 3.5 μmor less or approximately 5.0 μm or less, respectively.

A semiconductor oxidation apparatus satisfying such needs is proposed inZenno et al., “Development of New Oxidation Apparatus For ManufacturingSurface-Emitting Lasers”, Optical Alliance, pp.42-46, April 2004. FIG. 1shows a general structure of this proposed semiconductor oxidationapparatus. A semiconductor oxidation apparatus 1010 shown in FIG. 1 hasa sealable container (or oxidation chamber) 1012, and a heating stage1016 that has a built-in heater is provided in a chamber interior 1014of this oxidation chamber 1012. A substrate holder 1018 is provided onthe heating stage 1016, and a semiconductor sample (or semiconductorsubstrate) 1020 is placed on the substrate holder 1018. The oxidationchamber 1012 is also provided with an inlet pipe 1022 for supplying anoxidizing atmosphere including vapor into the chamber interior 1014, andan exhaust pipe 1024 for exhausting the oxidizing atmosphere within thechamber interior 1014 after an oxidation process ends. According to thesemiconductor oxidation apparatus 1010 having such a structure, it ispossible to uniformly oxidize the semiconductor sample 1020 with arelatively good reproducibility. However, the amount of oxidation of thesemiconductor sample 1020 is affected by inconsistencies among the lots,such as the composition and the film thickness after the crystal growthin a semiconductor film forming apparatus. Particularly in the case of asemiconductor layer including Al and As, the film thickness and the AlAscomposition are extremely sensitive to the oxidation temperature and thelike as described in Choquette et al., “Advances in Selective WetOxidation of AlGaAs Alloys”, IEEE Journal of Selected Topics in QuantumElectronics, Vol.3, No.3, pp.916-926, June 1997, and are also affectedby the thickness of a natural oxidation layer that is formed on a sidesurface of a non-oxidized layer of the semiconductor sample immediatelyprior to the oxidation process. As a result, the size of the currentconstricting part causes the inconsistency in the oscillationcharacteristic such as the optical output, and the yield deteriorates.Particularly in the case of a single-mode element in which the absolutevalue of the area of the current constricting part is small compared tothat of a multi-mode element, the effects of the inconsistency in theamount of oxidation on the inconsistency of the element characteristicsare extremely large, and if the area of the current constricting partbecomes large the element that should originally behave as a single-modeelement may behave like a multi-mode element.

In order to solve the problems described above, methods of monitoringthe amount of oxidation during the oxidation process have been proposedin Feld et al., “In Situ Optical Monitoring of AlAs Wet Oxidation Usinga Novel Low-Temperature Low-Pressure Steam Furnace Design”, IEEEPhotonics Technology Letters, Vol.10, No.2, pp.197-199, February 1998and a Japanese Laid-Open Patent Application No. 2003-179309. Accordingto the proposed methods, a semiconductor sample 1020 during theoxidation process is monitored by a microscope 1028 via a monitoringwindow 1026 as shown in FIG. 2. The oxidation distance or the area ofthe non-oxidized region (oxidation rate) is estimated from the contrastbetween the oxidized region and the non-oxidized region that aremonitored by the microscope 1028, and the amount of oxidation isthereafter controlled based on the estimated oxidation distance or thearea of the oxidation rate. But normally, the mesa diameter of the VCSELis approximately 10 μm to approximately 50 μm, and the magnification ofthe microscope 1028 must be set to approximately 1000 times in order tostrictly control the diameter of the current constricting part. Inaddition, in order to match the focal point of the microscope 1028 onthe mesa, a distance L1 between the semiconductor sample 1020 and themonitoring window 1026 and a distance L2 between the monitoring window1026 and the microscope 1028 must be set short as possible. However, ifthe distance L1 between the semiconductor sample 1020 and the monitoringwindow 1026 during the oxidation process is set short, localinconsistencies are generated in the vapor density distribution on thesemiconductor sample 1020 and the temperature distribution on thesemiconductor sample 1020, to thereby generate an in-plane distributionof the amount of oxidation and cause a deterioration in the yield. Onthe other hand, if the distance L2 between the monitoring window 1026and the microscope 1028 is set short, the index of refraction of themonitoring window 1026 may change due to the heat generated from theheater, and the optical elements such as a lens assembled in themicroscope 1028 may undergo a thermal deformation and generate a shiftin the focal point, to thereby deteriorate the measuring accuracy.

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide a novel anduseful semiconductor oxidation apparatus and method of producingsemiconductor element, in which the problems described above aresuppressed.

A more specific object of the present invention is to provide asemiconductor oxidation apparatus and a method of producingsemiconductor element, which can maintain an amount of oxidation of aselectively oxidizing layer included in a semiconductor sample uniformin an in-plane direction, and appropriately control the amount ofoxidation.

Still another object of the present invention is to provide asemiconductor oxidation apparatus comprising a sealable oxidationchamber defined by walls; a base provided within the oxidation chamberand configured to support a semiconductor sample; a supply partconfigured to supply water vapor into the oxidation chamber to oxidize aspecific portion of the semiconductor sample; a monitoring windowprovided in one of the walls of the oxidation chamber and disposed at aposition capable of confronting the semiconductor sample supported onthe base; a monitoring part provided outside the oxidation chamber andcapable of confronting the semiconductor sample supported on the basevia the monitoring window; and an adjusting part configured to adjust adistance between the base and the monitoring part. According to thesemiconductor oxidation apparatus of the present invention, it ispossible to adjust the temperature distribution and the vapor densitydistribution of the semiconductor sample uniform during the oxidationprocess, and the oxidation rate of the semiconductor sample can beaccurately monitored and evaluated at the time of the monitoring. Hence,it is possible to produce semiconductor elements having uniformcharacteristics.

In the semiconductor oxidation apparatus, the monitoring window may beprovided on one of the walls above the semiconductor sample supported onthe base, and the adjusting part may include an elevator mechanism thatis configured to move at least one of the base and the monitoring partupwards and downwards.

Alternatively, in the semiconductor oxidation apparatus, the adjustingpart may include a moving mechanism that is configured to move the basebetween a monitoring position where the semiconductor supported on thebase is adjacent to the monitoring window and a receded position wherethe semiconductor sample supported on the base is separated from themonitoring window by a longer distance that at the monitoring position.

A further object of the present invention is to provide a method ofproducing a semiconductor element by placing within a water vaporatmosphere a semiconductor sample that includes a mesa having asemiconductor layer including Al and As, and forming a currentconstricting part and a current injecting part that is surrounded by thecurrent constricting part in the semiconductor layer by oxidizing thesemiconductor layer from a peripheral end of the semiconductor layerappearing at an outer peripheral side surface of the mesa towards aninner radial direction so as to leave a central portion of thesemiconductor layer non-oxidized, the method comprising the steps of (a)interrupting an oxidation process at least once during oxidation of thesemiconductor layer; and (b) monitoring an oxidation rate of thesemiconductor layer while the oxidation process is interrupted.According to the method of the present invention, it is possible toaccurately evaluate the oxidation rate at the time when the oxidationprocess is interrupted, and produce semiconductor elements havinguniform characteristics.

The step (b) of the method may include (b1) moving the semiconductorsample within an oxidation chamber to a position where the mesa isadjacent to a monitoring part provided outside the oxidation chamber viaa monitoring window of the oxidation chamber while the oxidation processis interrupted; and (b2) obtaining the oxidation rate based on a size ofthe current constricting part or the current injecting part that ismonitored by the monitoring part. In this case, it is possible to obtainan extremely accurate oxidation rate.

The method may further comprise the steps of (c) obtaining an amount ofadditional oxidation that is to be made based on the oxidation rate; and(d) additionally oxidizing the semiconductor layer by the amount ofadditional oxidation. In this case, it is possible to positively adjustthe final amount of oxidation to a target value.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a general structure of anexample of a conventional oxidation apparatus in a vertical crosssection;

FIG. 2 is a cross sectional view showing a general structure of anotherexample of the conventional oxidation apparatus in a vertical crosssection;

FIG. 3 is a cross sectional view showing a structure of a firstembodiment of a semiconductor oxidation apparatus according to thepresent invention in a vertical cross section;

FIG. 4 is a functional block diagram showing a structure of a controlsystem of the semiconductor oxidation apparatus shown in FIG. 3;

FIG. 5A is a cross sectional view, in part, showing a semiconductorsample that is to be subjected to an oxidation process by thesemiconductor oxidation apparatus shown in FIG. 3 prior to theoxidation;

FIG. 5B is a cross sectional view, in part, showing the semiconductorsample that is subjected to the oxidation process by the semiconductoroxidation apparatus shown in FIG. 3 after the oxidation;

FIG. 6 is a flow chart for explaining a semiconductor element producingmethod using the semiconductor oxidation apparatus shown in FIG. 3;

FIGS. 7A through 7C are diagrams for explaining an oxidation processwith respect to an oxidizing portion;

FIG. 8 is a diagram for explaining an oxidation process with respect toan oxidizing part in conjunction with FIGS. 7A through 7C;

FIG. 9 is a cross sectional view showing a structure of a semiconductorlaser that is formed using the semiconductor sample after the oxidation;

FIG. 10 is a flow chart for explaining another semiconductor elementproducing method using the semiconductor oxidation apparatus shown inFIG. 3;

FIG. 11 is a diagram for explaining an origin restoring step;

FIG. 12 is a cross sectional view showing a structure of a secondembodiment of the semiconductor oxidation apparatus according to thepresent invention in a vertical cross section;

FIG. 13 is a cross sectional view showing a structure of a VCSELproduced by the first embodiment;

FIG. 14 is a diagram a wavelength dependency of reflectivities of theoxidized region and the non-oxidized region of the VCSEL produced by thefirst embodiment;

FIG. 15 is a cross sectional view showing a structure of a VCSELproduced by a second embodiment;

FIG. 16 is a plan view showing a VCSEL array chip using the VCSEL;

FIG. 17 is a diagram showing an optical transmission module using theVCSEL;

FIG. 18 is a diagram showing an optical communication module using theVCSEL;

FIG. 19 is a diagram showing a general structure of an image formingapparatus using the VCSEL; and

FIG. 20 is a plan view on an enlarged scale showing a light source forexposure in the image forming apparatus shown in FIG. 19.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will be given of various embodiments of the semiconductoroxidation apparatus according to the present invention and the method ofproducing semiconductor element according to the present invention, byreferring to FIG. 3 and the subsequent drawings.

First Embodiment

FIG. 3 is a cross sectional view showing a structure of a firstembodiment of the semiconductor oxidation apparatus according to thepresent invention in a vertical cross section. A semiconductor oxidationapparatus (hereinafter simply referred to as an oxidation apparatus) 10shown in FIG. 3 has a pressure-tight sealable container (or oxidationchamber) 12 made of a metal such as stainless steel. An internal space14 having a predetermined size is provided within the oxidation chamber12, and a base 16 for supporting a semiconductor sample is provided atthe bottom of the internal space 14.

In this embodiment, the base 16 is made up of a heating table 18, and asample table (or substrate holder) 22 that is provided on the heatingtable 18 and is designed to support thereon a semiconductor sample(substrate made of semiconductor wafer) 20 that is to be oxidized. Theheating table 18 has an electric heater 24 for heating the semiconductorsample 20 that is to be oxidized, and is freely rotatable and verticallymovable (that is, movable up and down) via a rotation and elevatormechanism (or moving mechanism and base moving mechanism) 26. The sampletable 22 is provided with a mechanism (not shown) for fixing thesemiconductor sample 20 that is to be oxidized on the sample table 22.The heating table 18 and the sample table 22 may have an integratedstructure.

The rotation and elevator mechanism 26 includes a driving shaft 30 thatextends vertically in the up and down directions and penetrates a bottomwall 28 of the oxidation chamber 12, a motor 32 that connects to anddrives the driving shaft 30 outside the oxidation chamber 12, and anelevator mechanism 34 that moves the motor 32 up and down together withthe driving shaft 30. In this embodiment, the elevator mechanism 34 isfixed to a fixing part 36 such as the floor. In addition, the elevatormechanism 34 is formed by a motor 38 and a rack-and-pinion (not shown),but may be formed by an air pressure or an oil pressure cylinder.Accordingly, the heating table 18 and the sample table 22 can rotate ina predetermined direction about the driving shaft 30 based on thedriving by the motor 32, together with the semiconductor sample 20 thatis fixed on the sample table 22, and can move up and down between anoxidizing position (receded position or lowered position) indicated by asolid line and a monitoring position (raised position) indicated by adotted line based on the driving by the elevator mechanism 34.

A top wall 40 of the oxidation chamber 12 has a monitoring window 42that is made of a light transmitting heat-resistant material, at aposition above the base 16 and confronting the semiconductor sample 20that is placed at the monitoring position. A camera 44 provided with amicroscope (hereinafter simply referred to as a microscope 44), whichforms a monitoring means, is provided above the monitoring window 42outside the oxidation chamber 12. The microscope 44 is desirably mountedon a fixing base 54 via a camera moving mechanism 52 that has 3 motors46, 48 and 50 for moving the microscope 44 in 2 mutually perpendiculardirections (X and Y directions) on the horizontal plane and in aperpendicular direction (Z direction) to the 2 directions (X and Ydirections), with an optic axis of the microscope 44 oriented in the upand down directions. The microscope 44 desirably has an automaticfocusing function so that the focal point can be automatically adjustedwith a precision on the μm order. In order to minimize the heat that isradiated to the outside via the monitoring window 42 during theoxidation process and to prevent damage to the microscope 44 (thermaldeformation of optical parts such as a lens) due to the heat radiatedvia the monitoring window 42, it is possible to provide a heat blockingmechanism 60. The heat blocking mechanism 60 includes a heat blockingplate 56 that opens and closes to expose and cover the monitoring window42 on the outer side or the inner side of the monitoring window 42, anda motor 58 or a cylinder that moves the heat blocking plate 56 to openand close. In order to improve the heat blocking effect, the heatblocking plate 56 may be made of a material having a high heatinsulating efficiency or, have a surface confronting the internal space14 made of a reflecting material or processed to be reflecting.

The oxidation chamber 12 is provided with 3 pipes 62, 64 and 66, whichpenetrate the walls of the oxidation chamber 12, for adjusting theatmosphere within the internal space 14. The first pipe 62 supplies anoxidization atmosphere gas to the internal space 14. The first pipe 62branches into two outside the oxidation chamber 12, with one branchingpipe 68 connecting to a water vapor supply source (or water vapor supplypart) 72 via a solenoid-operated valve (hereinafter simply referred toas a solenoid valve) 70, and another branching pipe 74 connecting to anitrogen supply source (or nitrogen supply part) 78 via a solenoid valve76. Hence, an oxidation gas that is a mixture of the water vaporsupplied from the vapor supply source 72 and the nitrogen supplied fromthe nitrogen supply source 78 is supplied to the internal space 14. Asshown in FIG. 3, the outlet end of the first pipe 62 is desirablydirected towards,a space above the semiconductor sample 20 that isplaced at the oxidizing position, so that it is possible to sufficientlysupply the oxidation gas to the semiconductor sample 20 during theoxidation process. As will be described later, the second pipe 64supplies a low-temperature nitrogen gas, as an inert gas, for replacingthe atmosphere within the internal space 14 to nitrogen and for coolingthe semiconductor sample 20, when the oxidation process is interrupted.The second pipe 64 is connected to a low-temperature nitrogen gassupplying source (or low-temperature nitrogen gas supply part) 82 via asolenoid valve 80. For the purpose of cooling the semiconductor sample20, the outlet end of the second pipe 64 is desirably directed to thesemiconductor sample 20 at the oxidizing position or to a space abovethe semiconductor sample at the oxidizing position, so that the suppliedlow-temperature nitrogen gas is sprayed or blasted on the semiconductorsample 20. The third pipe 66 exhausts the residual oxidation gas withinthe internal space 14 when the oxidation process is interrupted. Thethird pipe 66 is connected to a vacuum source 86 via a solenoid valve84. A fourth pipe 88 further penetrates the wall of the oxidationchamber 12. This fourth pipe 88 is connected to a solenoid valve 90 forflow. By maintaining the solenoid valve 90 in an open state during theoxidation process, it is possible to push out via the fourth pipe 88 anamount of water vapor gas in accordance with the supplied amount, whilemaintaining the flow or circulation state of the water vapor within theinternal space 14.

FIG. 4 is a functional block diagram showing a structure of a controlsystem 100 of the oxidation apparatus 10 shown in FIG. 3. As shown inFIG. 4, the control system 100 includes a control apparatus 102. Thiscontrol apparatus 102 includes a central processing unit (CPU) 104. TheCPU 104 is connected to an image processing part 106 that processes dataof an image picked up by the microscope 44, a display part (or monitor)106 that displays the image picked up by the microscope 44, the motors32 and 38 of the rotation and elevator mechanism 26, the motors 46, 48and 50 of the camera moving mechanism 52 that move the microscope 44 inthe X, Y and Z directions, a motor control part 110 that controls themotor 58 of the heat blocking mechanism 60, a solenoid valve controlpart 112 that controls the solenoid valves 70, 76, 80, 84 and 90, and aheater control part 114 that controls the heater 24 by PID. The controlapparatus 102 also has a storage part 116 that stores programs,information and the like necessary to execute the oxidation process(including an interruption process) that will be described later, and anoperation part 118 that executes operation processes that will bedescribed later.

When forming an oxidized layer of the VCSEL using the oxidationapparatus 10, the semiconductor sample 20 prior to the oxidation processhas a cross sectional structure shown in FIG. 5A, for example. The crosssectional structure shown in FIG. 5A includes an n-electrode 120, ann-GaAs substrate 122, and a lower semiconductor distributed Braggreflection (DBR) mirror 124 in this order from the bottom, and a mesa126 is supported on the lower semiconductor distributed Bragg reflection(DBR) mirror 124 at a constant density (at predetermined intervals inthe X and Y directions). The mesa 126 includes an active region 128, anupper semiconductor distributed Bragg layer 132, and a contact layer 134in this order from the bottom. The upper semiconductor distributed Bragglayer 132 includes a selectively oxidizing layer 130 which is to beselectively oxidized. The selectively oxidizing layer 130 includes Aland As, and is partially -oxidized after this layer 130 is selectivelyoxidized as will be described later. In addition, the mesa 126, whenviewed from the top towards the bottom in FIG. 5A, has a circular shape,an oval shape, a quadrangle shape (square or rectangular shape) or apolygonal shape other that the quadrangle shape.

FIG. 6 shows an oxidation process for oxidizing a selectively oxidizinglayer 130 of the semiconductor sample 20 having the cross sectionalstructure described above using the oxidation apparatus 10. Theoxidation process shown in FIG. 6 generally includes a primary oxidationprocess #10 for carrying out a primary oxidation with respect to theselectively oxidizing layer 130, an evaluation process #20 fortemporarily stopping the oxidation from progressing and evaluating theamount of oxidation (or oxidation rate), and a secondary oxidationprocess #30 for oxidizing the selectively oxidizing layer 130 to a finaltarget amount of oxidation based on the evaluation result of the amountof oxidation.

In the primary oxidation process #10, the semiconductor sample 20, whichhas the cross sectional structure described above and has not yet beenoxidized, is placed and fixed on the sample table 22 by a preparationstep #11. In this state, the base 16 including the sample table 22 isfixed to the oxidizing position (or lowered position). If necessary, thecontrol apparatus 102 opens the exhausting solenoid valve 84 via thesolenoid valve control part 112, and exhausts the residual atmospherewithin the internal space 14. The other solenoid valves 70, 76 and 80are closed in this state. Further, if necessary, the control apparatus102 drives the motors 46, 48 and 50 of the camera moving mechanism 52,and moves the microscope 44 to a position separated from the monitoringwindow 42 or, to a position receded from the region confronting themonitoring window 42. If the heat blocking plate 56 for exposing andcovering the monitoring window 42 is provided, the motor 58 is driven tomove the heat blocking plate 56 to a closed position to cover themonitoring window 42.

Next, the control apparatus 102 closes the exhausting solenoid Valve 84and the low-temperature nitrogen gas supplying solenoid valve 80 andopens the water vapor supplying solenoid valve 70 and the nitrogensupplying solenoid valve 76, to supply the mixture gas of water vaporand nitrogen to the internal space 14 by a solenoid valve control step#12. In addition, while the mixture gas is supplied to the internalspace 14, the control apparatus 102 opens the solenoid valve 90, so thata circulation state of the water vapor gas within the internal space 14of the oxidation chamber 12 is formed, and at the same time, a state isformed such that the water vapor gas is pushed out via the pipe 88.Instead of opening the solenoid valve 90, it is possible to open thesolenoid valve 84 to exhaust the water vapor gas from the internal space14 of the oxidation chamber 12, and at the same time, maintain theinternal space 14 to a constant pressure (decompression state oratmospheric pressure state), so as to form the circulation state of thewater vapor gas within the internal space 14. Next, the controlapparatus 102 turns the heater 24 ON via the heater control part 114 bya heater control step #13. Hence, an oxidation gas made up of watervapor and nitrogen fills the internal space 14 of the oxidation chamber12, and the internal space 14 is adjusted to a temperature statenecessary for the oxidation of the semiconductor sample 20, so as tostart the oxidation of the selectively oxidizing layer 130 of thesemiconductor sample 20 by a primary oxidation step #14. The oxidationof the selectively oxidizing layer 130 progresses from the peripheralend of the selectively oxidizing layer 130 exposed at the outerperipheral side surface of the mesa 126 in a radial direction towardsthe inner side. For example, in a case where the mesa 126 has a circularflat shape, an oxidized region 140 gradually progresses from theperipheral end towards the center, and a circular non-oxidized region142 having a shape similar to the circular flat shape of the mesa 126remains on the inner side the ring-shaped oxidized region 140, as may beseen from FIGS. 7A through 7C.

The control apparatus 102 judges the end of the primary oxidation by aprimary oxidation end judging step #15. More particularly, in a casewhere the entire oxidation process is completed in a state where thenon-oxidized region 142 having a predetermined size remains at thecenter (final oxidized state) as shown in FIG. 8, the oxidation isinterrupted before a primary oxidized region 140A which is oxidized bythe primary oxidation reaches the final oxidized state (final oxidizedposition 144), that is, by leaving a slight or small secondary oxidizingregion (or additionally oxidizing region) 140B. The timing at which theoxidation is interrupted (primary oxidation time) may be obtained fromexperience based on the time it takes to complete the oxidation. Forexample, the control apparatus 102 stores in the storage part 116 aprimary oxidation time t1 (=t−Δt) which is obtained by subtracting apredetermined time Δt from a total required oxidation time t that isobtained from experience, and judges the end of the primary oxidation ata time when the elapsed time from the start of the oxidation reaches theprimary oxidation time t1. When the control apparatus 102 judges the endof the primary oxidation, the control apparatus 102 closes the watervapor supplying solenoid valve 70 and the nitrogen supplying solenoidvalve 76 by a solenoid valve control step #16. The control apparatus 102stops the heater 24 before the solenoid valves 70 and 76 are closed or,after the solenoid valves 70 and 76 are closed, by a heater control step#17. Then, the control apparatus 102 opens the rapid-coolinglow-temperature nitrogen gas supplying solenoid valve 80, and replacesatmosphere within the internal space 14 by nitrogen to cool the internalspace 14 by a solenoid valve control step #18. Since the outlet end ofthe low-temperature nitrogen gas supplying pipe 64 is directed towardsthe semiconductor sample 20 as described above, the low-temperaturenitrogen gas supplied from the pipe 64 is directly sprayed or blastedonto the semiconductor sample 20, to thereby virtually stop orcompletely stop the progress of the oxidation. Instead of supplying thelow-temperature nitrogen gas, it is possible to open the exhaustingsolenoid valve 84 and exhaust the water vapor from the internal space14. In this case, the progress of the oxidation similarly stops. Inaddition, while the oxidation is interrupted, in a state where thesupply of the water vapor and nitrogen is stopped by closing the watervapor supplying solenoid valve 70 and the nitrogen supplying solenoidvalve 76, the circulation state of the nitrogen gas within the internalspace 14 may be maintained by opening the low-temperature nitrogen gassupplying solenoid 80 and the flow solenoid valve 90 or, nitrogen may beexhausted while maintaining the internal space 14 to a constant pressureby the vacuum source 86 by opening the low-temperature nitrogen gassupplying solenoid valve 80 and the exhausting solenoid valve 84instead.

Next, in the evaluation process #20, in a case where the oxidationapparatus 10 has the heat blocking mechanism 54 including the heatblocking plate 56, the control apparatus 102 drives the motor 58 andmoves the heat blocking plate 56 from above the monitoring window 42 toan open position so as to expose the monitoring window 42. In a casewhere the oxidation apparatus 10 has the camera moving mechanism 52 andthe microscope 44 is at the receded position separated from themonitoring window 42 or the oxidation chamber 12 during the oxidationprocess, the control apparatus 102 drives the camera moving mechanism 52and moves the microscope 44 to the position confronting the monitoringwindow 42. These operations of the control apparatus 102 with respect tothe heat blocking mechanism 54 and the camera moving mechanism 52 aremade in a preparation step #21. Then, the control apparatus 102 drivesthe elevator motor 38 to raise the base 16 to the monitoring position sothat the semiconductor sample 20 confronts the microscope 44 via themonitoring window 42 by a sample raising step #22. The control apparatus102 further starts the microscope 44 to acquire an enlarged image of themesa 126 by an image pickup (or photo taking) step #23. Because themicroscope 44 has an automatic focusing mechanism, the microscope 44automatically matches the focal point (that is, focuses) on the mesa126. Accordingly, as shown in FIG. 8, a boundary between the oxidizedregion 140 that has been oxidized by the primary oxidation and thenon-oxidized region can be clearly recognized on the acquired enlargedimage. The data of the image picked up by the microscope 44 are suppliedto the image processing part 106 wherein necessary image processings aremade. The image data after the image processing are stored in thestorage part 116 if necessary. In addition, the enlarged image of themesa 126 is displayed on the display part 108 using the image data afterthe image processing. Moreover, the image data after the imageprocessing are supplied to the operation part 118 wherein the amount ofoxidation (oxidation time t2) of the secondary oxidation is calculatedby an evaluation step #24.

A description will be given of an example of the calculation made in theoperation part 118, by referring to FIG. 8. The oxidation rate isrepresented by a distance d from the outer peripheral end of thecircular mesa 126 to the inner peripheral end of the ring-shapedoxidized region 140 along an imaginary line passing the center of thecircular mesa 126, for example. The operation part 118 calculates theamount (=Δd2) to be oxidized by the secondary oxidation, from theoxidation rate (=distance d1) and the size (distance from the outerperipheral end of the circular mesa 126 to the final oxidized position144) of the non-oxidized region 142 which is the final target. Then, theoperation part 118 divides the amount of oxidation (distance d1) by thetime t1 required for the primary oxidation, and obtains the amount ofoxidation per unit time (oxidation rate coefficient α=d1/t1). Finally,the operation part 118 obtains the time required for the secondaryoxidation (that is, the secondary oxidation time) from t2=d2/α.

The control apparatus 102 judges the end of the evaluation process #20by an end judging step #25.

Next, in the secondary oxidation process #30, the preparations necessaryfor the secondary oxidation are made in a preparation step #31. Forexample, the control apparatus 102 drives the motors 46, 48 and 50 ofthe camera moving mechanism 52, and moves the microscope 44 to theposition separated from the monitoring window 42 or, to a positionreceded from the region confronting the monitoring window 42. If theheat blocking plate 56 for exposing and covering the monitoring window42 is provided, the motor 58 is driven to move the heat blocking plate56 to the closed position to cover the monitoring window 42. Then, thecontrol apparatus 102 closes the low-temperature nitrogen supplyingsolenoid valve 80 by a valve control step #32. If the exhaustingsolenoid valve 84 is opened instead of supplying the low-temperaturetemperature nitrogen gas, this exhausting solenoid valve 80 is closed.In addition, the motor 38 of the elevator mechanism 34 is driven tolower the base 16 to the oxidizing position by a sample lowering step#33. The control apparatus 102 then opens the water vapor supplyingsolenoid valve 70 and the nitrogen supplying solenoid valve 76, andsupplies the mixture gas of water vapor and nitrogen to the internalspace 14 by a solenoid valve control step #34. In addition, in a casewhere the heater 24 is stopped at the time when the primary oxidationprocess #10 ends, the control apparatus 102 again drives the heater 24by a heater control step #35. As a result, the oxidation of theselectively oxidizing layer 130 is restarted, and the oxidized region140 is enlarged towards the inner side while the non-oxidized region 142becomes smaller. The duration (time) of the secondary oxidation process#30 is measured, and when the measured time reaches the secondaryoxidation time t2 described above, the control apparatus 102 judges theend of the secondary oxidation by a secondary oxidation end judging step#36. Finally, when the control apparatus 102 judges the end of thesecondary oxidation, the control apparatus 102 closes the water vaporsupplying solenoid valve 70 and the nitrogen supplying solenoid valve 76by a solenoid valve control step #36, and stops the heater 24 by aheater control step #37, to thereby end the secondary oxidation process#30. When ending the oxidation process, it is desirable to open thelow-temperature nitrogen gas supplying solenoid valve 80 and spray orblast the low-temperature nitrogen gas onto the semiconductor sample 20.

As shown in FIG. 5B and 8, the non-oxidized region 142 having thepredetermined size and the oxidized region 140 surrounding the peripheryof the non-oxidized region 142 are formed on the mesa 126 after thesecondary oxidation process #30 ends. The non-oxidized region 142 andthe oxidized region 140 respectively become a current injecting part 150and a current constricting part 152. The semiconductor sample 20 that isformed with the current injecting part 150 and the current constrictingpart 152 is thereafter subjected to the necessary thin-film formingprocess, to form a VCSEL 160 shown in FIG. 9. This VCSEL 160 includes aninsulator layer 154 made of SiO₂ or the like, an insulator layer 156made of a polyimide or the like, and a p-electrode 158.

According to the oxidation apparatus 10 of this embodiment, theselectively oxidizing layer 130 of the semiconductor sample 20 isoxidized to a certain extent by the primary oxidation process, and theamount of oxidation (oxidation rate) of the primary oxidation isthereafter obtained in a state where the progress of the oxidation istemporarily stopped, so as to determine the amount of oxidation(oxidation time) of the secondary oxidation process that remains to bemade by feeding back the result of the primary oxidation process.Accordingly, the size of the current injecting part that is finallyformed is virtually the target size or is exactly the target size. Inorder to match the amount of oxidation at the time when the secondaryoxidation ends to the target amount of oxidation, it is desirable thatthe ratio (=d1/(d1+d2)) (%) of the amount of primary oxidation (d1) withrespect to the total amount of oxidation (=d1+d2) takes a value that isas large as possible. For example, the ratio (=d1/(d1+d2) (%) is 80% to95%, and preferably 85% to 95%, and more preferably 90% to 95%.

In the description given above, it is assumed that the mesa 126 has theflat circular shape, and the amount of oxidation is obtained in terms ofthe mesa diameter (d1, d2). However, the amount of oxidation may also beevaluated by the area. In this case, an oxidation rate coefficientα(=(area of selectively oxidizing area prior to oxidation)−((area ofnon-oxidized region at the time when the primary oxidationends)/(oxidation time)) may be obtained from the ratio of thenon-oxidized area at the time when the oxidation ends with respect tothe area of the final target non-oxidized region, so as to obtain theamount of oxidation (oxidation time) of the secondary oxidation based onthe oxidation rate coefficient α.

[First Modification]

In the first embodiment described above, the base 16 and thesemiconductor sample 20 are fixed to a predetermined position during theoxidation. However, in a first modification of the first embodiment ofthe present invention, the rotation mechanism (rotation and elevatormechanism 26) of the base 16 may be driven to rotate the semiconductorsample 20 during the oxidation. In this case, the spatial position ofeach mesa 126 changes with time, and it is possible to uniformly oxidizethe selectively oxidizing layer 130 of each mesa 126, to thereby improvethe yield of the VCSEL 160.

[Second Modification]

In the first embodiment described above, the secondary oxidation processis carried out based on the evaluation result of the primary oxidationprocess, and the size of the final non-oxidized region (currentinjecting part) is not confirmed. However, in a second modification ofthe first embodiment, the size of the non-oxidized region is measuredafter the secondary oxidation process ends, and based on the measuredsize of the non-oxidized region, a third oxidation process isadditionally carried out if necessary. In this case, the size of thenon-oxidized region is measured after the secondary oxidation process,and based on the measured result, an amount of oxidation of the thirdoxidation process and the oxidation time required for the thirdoxidation process are calculated similarly as described above, to carryout the third oxidation process based on the calculated amount ofoxidation and oxidation time.

It is preferable that the amount of oxidation for the primary oxidationprocess and the secondary oxidation process are evaluated for the samemesa. When not rotating the base 16 and the semiconductor sample 20during the oxidation, the measurement of the amount of oxidation for theprimary oxidation process and the amount of oxidation for the secondaryoxidation process can be measured for the same mesa by matching thefocal point of the microscope 44 (that is, the camera) to apredetermined plane coordinate (XY coordinate).

When rotating the base 16 and the semiconductor sample 20 during theoxidation, the mesa that is the evaluation target and located at thefocal point of the microscope 44 at the time when the primary oxidationprocess ends may have moved at the time when the secondary oxidationprocess ends, and in this case, the evaluation target mesa is notlocated at the focal point of the microscope 44 at the time when thesecondary oxidation process ends. In order to eliminate this problem,another semiconductor element producing method may be employed as shownin FIG. 10. FIG. 10 is a flow chart for explaining this othersemiconductor element producing method using the semiconductor oxidationapparatus shown in FIG. 3. In FIG. 10, those steps that are the same asthose corresponding steps in FIG. 6 are designated by the same referencenumerals, and a description thereof will be omitted. In FIG. 10, anorigin restoring step #19 is carried out to restore the base 16 to theposition of the origin after the primary oxidation ends, and an originrestoring step #38 is carried out to restore the base 16 to the positionof the origin after the secondary oxidation ends.

In order to carry out an origin restoring step, it is preferable to usefor the motor 32 that rotates the base 16 a servo motor having anencoder provided with an origin position (Z-phase). More particularly, adescription will be given of the origin restoring step using the Z-phaseof the servo motor. Suppose that the focal point of the microscope 44(that is, the camera) exists at a position (r, θ) moved by apredetermined distance (x0, y0) in the X and Y directions from a center(origin of the XY coordinate system) of the driving shaft 30 as shown inFIG. 11, that the evaluation target mesa 126 is located at this camerafocal point, and that the servo motor is stopped in a state where apredetermined number of pulses have been output from the Z-phase outputat a time when the primary oxidation process or the secondary oxidationprocess starts. In order to simplify matters for the sake ofconvenience, it is assumed that the predetermined number of pulses iszero. Under these conditions, suppose that the evaluation target mesa126 has stopped at a coordinate (r, θ+θ′) moved by an angle θ′ from thecamera focal point at the time when the primary or secondary oxidationprocess ends. In this case, the control apparatus 102 stores a numberN_(θ) of pulses from a time when the Z-phase of the servo motor isoutput to a time when the rotation thereof stops. The number N_(θ) ofpulses corresponds to the moved angle θ″. Accordingly, in the originrestoring step, the control apparatus 102 calculates the moved angle θ′from the number N_(θ) of pulses output from the servo motor, andcalculates an angle (360°-θ′) required to rotate and restore the rotaryshaft of the servo motor to the angular position at the time prior tothe oxidation process and a number N_((360°-θ)) of pulses correspondingto the angle (360°-θ′). In addition, the control apparatus 102 rotatesthe rotary shaft of the servo motor until the number N_((360°-θ)) ofpulses is output from the servo motor, so as to move the evaluationtarget mesa 126 to within a camera field (or field of view) 134.Therefore, the amount of oxidation after the primary oxidation processand the secondary oxidation process can be evaluated with respect to thesame mesa 126.

Another simple method may be used in place of the above method using theZ-phase of the servo motor. In other words, a detection part (forexample, a projection, cutout, read mark, magnet, and the like) that isto be detected is mounted on the driving shaft 30 or on another rotarymember (for example, gear) that is linked to be driven by the drivingshaft 30. On the other hand, a detector is provided in a vicinity of thedetection part. In the origin restoring step prior to the evaluationprocess, the driving shaft 30 is rotated until the detector detects thedetection part, so as to restore the rotary position of the drivingshaft 30. The detector may use optical detection, a mechanicaldetection, an electromagnetic detection or a detection employing anarbitrary combination of such detections.

In order to positively move the evaluation target mesa 126 within thecamera field, the origin restoring step preferably rotates the base 16and the driving shaft 30 at a speed that is slower than that at the timeof the oxidation. Accordingly, a motor that can switch its rotationalspeed between a high speed and a low speed is preferably used for themotor 32 that rotates the base 16.

[Third Modification]

In the first embodiment described above, the base 16 is formed byintegrally combining the heating table 18 and the sample table 22.However, in a third modification of the first embodiment, the heatingtable 18 is fixed to the oxidation chamber 12, and only the sample table22 is movable between the oxidizing position (lowered position) and themonitoring position (raised position) with respect to the heating table18. In this case, as shown in FIG. 12, the driving shaft 30 penetrateswithin the heating table 18, and the sample table 22 is supported onthis penetrating driving shaft 30. According to this oxidation apparatus10 having the structure shown in FIG. 12, the semiconductor sample 20can be separated from the heater 24 when interrupting the oxidation andafter the oxidation. Consequently, the cooling speed of thesemiconductor sample 20 becomes fast, and the oxidation can beinterrupted and ended positively. In addition, the amount of oxidationwithin the semiconductor sample 20 can be maintained uniform and to atarget value.

[Fourth Modifications]

According to the first embodiment and the first modification thereof,the semiconductor sample and the table that supports the semiconductorsample are raised towards the monitoring window that is provided at anupper portion of the oxidation chamber when interrupting the oxidation.But in this fourth modification of the first embodiment, a monitoringroom is provided on the side of the oxidation chamber, and thesemiconductor sample and the sample table that supports thesemiconductor sample are moved horizontally to the monitoring room whenmonitoring the semiconductor sample. A monitoring means may be providedat a ceiling part of the monitoring room, so as to monitor and evaluatethe amount of oxidation of the semiconductor sample from above. In thiscase, the height of the ceiling part of the monitoring room, thatconfronts the sample table that moves horizontally from the oxidationchamber to the monitoring room, is preferably set low so that themonitoring means may monitor the semiconductor sample from a closelyadjacent position. In addition, although the embodiment and themodifications described above use the solenoid valves to control thesupply and exhaust of the water vapor, nitrogen and the like, it is ofcourse possible to use other types of valves instead.

FIG. 13 is a cross sectional view showing a structure of a VCSELproduced by the first embodiment. A VCSEL 200 shown in FIG. 13 outputs alaser oscillation having a wavelength of 1.3 μm, and has an n-GaAssubstrate 202 having a <100> face orientation of 3-inch size. Ann-Al_(x)Ga_(1-x)As (x=0.9) layer and an n-GaAs layer are alternatelystacked for 35.5 periods on the substrate 202, to form a periodicstructure having a thickness that is ¼ the oscillation wavelength withinthe medium and forming an n-semiconductor distributed Bragg reflectionmirror (hereinafter referred to as a lower semiconductor DBR mirror orsimply lower DBR mirror) 204. A multi-quantum well active region 212,including an undoped lower GaAs spacer layer 206 and an undoped upperGaAs spacer layer 214, is formed on the lower DBR mirror 204. Themulti-quantum well active region 212 further includes GaInNAs welllayers 208 and GaAs barrier layers 210 that are alternately stackedbetween the undoped upper and lower GaAs spacer layers 214 and 206.There are 3 GaInNAs well layers 208 and 2 GaAs barrier layers 210 in themulti-quantum well active region 212.

A p-semiconductor distributed Bragg reflection mirror (hereinafterreferred to as an upper semiconductor distributed DBR or simply upperDBR mirror) 216 is formed on the spacer layer 214. A C-dopedp-Al_(x)Ga_(1-x)As (x=0.9) layer and a p-GaAs layer are alternatelystacked for 25 periods, for example, to form a periodic structure havinga thickness that is ¼ the oscillation wavelength within the medium andforming the upper DBR mirror 216. A selectively oxidizing layer 218 madeof AlAs and having a thickness of 30 nm, for example, is formed at alower part within the upper DBR mirror 216. A GaAs contact layer 220 atan uppermost part of the upper DBR mirror 216 also functions as acontact layer for making contact with the electrode. The In compositionx is 33% and the nitrogen composition is 1.0% for the well layer 208within the active region 212. The well layer 208 has a thickness of 7nm, and has a compression distortion (high distortion) of approximately2.1% with respect to the substrate 202.

MOCVD is used for the thin-film forming method. H₂ is used for a carriergas, and trimethylgallium (TMG), trimethylindium (TMI) and arsine (AsH₃)are used as raw materials for the GaInNAs well layer 208.Dimethylhydrazine (DMHy) is used as a raw material for the nitrogen.Since DMHy dissolves at a low temperatures, it is suited forlower-temperature growth at temperature of 600° C. or lower, and isparticularly suited for growing the well layer of the multi-quantum wellactive region having a large distortion that requires thelow-temperature growth. In a case where the distortion of the well layerof the multi-quantum well active region in the GaInNAs VCSEL is large,it is preferable to use the low-temperature growth that becomesunbalanced. In this embodiment, the GaInNAs well layer 208 is grown at540° C.

A mesa 222 is formed by wet etching or a dry etching such as RIE, RIBEand ICP, in a state where at least the side surface of the p-AlAsselectively oxidizing layer 218 is exposed. The mesa 222, when viewedfrom the top towards the bottom in FIG. 13 has a square shape.Thereafter, the oxidation apparatus 10 described above is used to forman AlxOy current constricting part by oxidizing the AlAs selectivelyoxidizing layer 218 from exposed side surface by use of water vapor. Inthis state, the wafer (sample) is placed on the sample table (substrateholder), the base is moved to the oxidizing position, and predeterminedwater vapor is supplied. Since the monitoring window is separated fromthe wafer, the oxidation is made uniformly. The wafer is then raised toa predetermined oxidation temperature of approximately 400° C., and theprimary oxidation is started. The oxidation is interrupted beforereaching an anticipated time when the desired amount of oxidation willbe reached. Thereafter, the heating table having the wafer placedthereon is moved to the monitoring position that is close to themonitoring window, and the oxidizing distance, oxidizing area, thevertical horizontal lengths of the non-oxidized region, and the area ofthe non-oxidized region are monitored by the microscope. Based on theinformation obtained by the monitoring of the microscope, the oxidationspeed of the primary oxidation and the oxidizing distance required forthe secondary oxidation (additional oxidation) are calculated, and theadditional oxidation time is obtained. The wafer is returned to theoxidizing position, and the secondary oxidation (additional oxidation)is carried out. When the oxidation time of the secondary oxidationelapses, the supply of water vapor is stopped and low-temperaturenitrogen is sprayed or blasted onto the wafer, and the oxidation isended by stopping the heater.

By the oxidation process described above, it was confirmed by thepresent inventors that the area of the current injecting part can becontrolled with a high accuracy in a state where the surface uniformityof the oxidation rate on the wafer is satisfactorily maintained. Theoxidation may be interrupted or ended by stopping the supply of watervapor, but it is preferable to lower the substrate temperature (sampletemperature) in order to more positively interrupt or end the oxidation.In addition, although the oxidation process is divided into 2 stages inthe embodiment and modifications described above, it is of coursepossible to divide the oxidation process into 3 or more stages.

After the oxidation process ends, the mesa 222 is protected by aninsulator layer 224 made of SiO₂ or the like, and thereafter, the etchedportion is filled with polyimide 226 and planarized. Then, the p-contactlayer 220 and the polyimide 226 and the insulator layer 224 existingabove the upper mirror layer where the light emitting portion isprovided are eliminated, so as to form a p-electrode 228 on thep-contact layer 220 at a portion other than the light emitting portion.In addition, an n-electrode 230 is formed on the back surface of thesubstrate 202.

The present inventors have confirmed the following with respect to theVCSEL that is made in the above described manner. The oscillationwavelength of the VCSEL was approximately 1.3 μm. In addition, since thecurrent constricting part was formed by selective oxidation of theselectively oxidizing layer having Al and As as the main components, alow threshold current was obtained. According to the currentconstricting structure using the current constricting part that is madeof the Al oxide layer obtained by selective oxidation of the selectivelyoxidizing layer, the spreading of the current was suppressed by formingthe current constricting part close to the active region, and it waspossible to efficiently trap the carrier in a small region that is notexposed to the atmosphere. In addition, since the Al oxide layer wasformed by oxidation, the index of refraction became small, and it waspossible to efficiently trap the light in the small region in which thecarrier is trapped by the convex lens effect, to thereby enable loweringof the threshold current. Moreover, since the current constrictingstructure can be formed with ease, the production cost of the VCSEL wasreduced. Furthermore, since the oxidation progresses at the positionseparated from the monitoring window, it was possible to evaluate theoxidation rate by temporarily stopping the oxidation and to additionallycarry out the oxidation depending on the evaluation result, the currentinjecting part was uniformly formed to the target size for each mesa, tothereby improve the yield.

The monitoring of the oxidized region was made by receiving thereflected light of the light emitted from a light source such as atungsten lamp to a CCD camera or a vidicon camera via the microscope,and displaying an enlarged image of the mesa on the display part such asa television monitor. On the displayed image of the mesa, the boundarybetween the oxidized region and the non-oxidized region was clearlyrecognizable due to the difference in the reflectivities.

FIG. 14 is a diagram a wavelength dependency of reflectivities of theoxidized region and the non-oxidized region of the VCSEL of the firstembodiment. In FIG. 14, the reflectivity of the oxidized region isindicated by a dotted line, and the reflectivity of the non-oxidizedregion is indicated by a solid line. As may be seen from FIG. 14, thereis not much difference between the reflectivities of the oxidized regionand the non-oxidized region in a high reflection band in a vicinity ofthe wavelength of 1.3 μm. However, the reflectivities of the oxidizedregion and the non-oxidized region in bands on both sides of the highreflection band are clearly different. As the wavelengths to bemonitored, the short wavelength side of the high reflection band is notpreferable since the light absorption due to the DBR mirror materialoccurs, but the light absorption problem does not occur on the longwavelength side of the high reflection band, and the reflectivity of theoxidized region is high. Accordingly, it is preferable to use a camerahaving a good sensitivity in the wavelengths on the long wavelength sideof the high reflection band. Of various image pickup devices that arepresently available, a CCD camera having good sensitivity in the nearinfrared region may be used to monitor the VCSEL that has the shortoscillation wavelength. If the oscillation wavelength of the VCSEL is1.3 μm as in this embodiment, an infrared vidicon camera having goodsensitivity in the long wavelength side of the high reflection band maybe used for the monitoring. In addition, the so-called cofocal lasermicroscope which makes the san by use of infrared laser light andreceives the scanned result by a camera is preferable from the point ofview that the resolution can be set extremely high, and the laserwavelength may be appropriately selected for use.

Second Embodiment

FIG. 15 is a cross sectional view showing a structure of a VCSELproduced by a second embodiment of the present invention. A VCSEL 300shown in FIG. 15 outputs a laser oscillation having a wavelength of 780nm, and has an n-(100)GaAs substrate 302 having an inclination angle of2° in a direction of the <100> face orientation. An n-Al_(0.9)Ga_(0.1)Aslayer and an n-Al_(0.3)Ga_(0.7)As layer are alternately stacked for 35.5periods on the substrate 302, to form a periodic structure having athickness that is ¼ the oscillation wavelength within the medium andforming an n-semiconductor distributed Bragg reflection mirror(hereinafter referred to as a first reflection mirror or simply n-DBRmirror) 304. Between two mutually adjacent n-Al_(0.9)Ga_(0.1)As andn-Al_(0.3)Ga_(0.7)As layers, there is inserted a composition inclinedlayer (not shown) having a thickness of 20 nm and in which the Alcomposition gradually changes from the Al composition of then-Al_(0.9)Ga_(0.1)As layer to the Al composition of then-Al_(0.3)Ga_(0.7)As layer. The composition inclined layer is alsosometimes referred to as a composition modulated layer or a compositiongradation layer. The thickness of the periodic structure, including thecomposition inclined layers, is set to ¼ the oscillation wavelengthwithin the medium. Due to this structure, when a current is applied tothe n-DBR mirror 304, it is possible to smoothen the band discontinuitybetween the n-Al_(0.9)Ga_(0.1)As layer and the n-Al_(0.3)Ga_(0.7)Aslayer and suppress the resistance from becoming high. A quantum wellactive region 308, including an Al_(0.5)Ga₀₅As lower spacer (cladding)layer 306 and an Al_(0.5)Ga_(0.5)As upper spacer (cladding) layer 310,is formed on the n-DBR mirror 304. The quantum well active region 308which makes the wavelength 780 nm further includes AlGaAs well layersand Al_(0.3)Ga_(0.7)As barrier layers that are alternately stackedbetween the upper and lower spacer layers 310 and 306, and there are 3AlGaAs well layers and 2 Al_(0.3)Ga_(0.7)As barrier layers. On thespacer layer 310, p-Al_(x)Ga_(1-x)As (x=0.9) and p-Al_(x)Ga_(1-x)As(x=0.3) layers are alternately stacked for 25 periods, for example, toform a periodic structure that forming an n-semiconductor distributedBragg reflection mirror (hereinafter referred to as a second reflectionmirror or simply p-DBR mirror) 312. Between two mutually adjacentp-Al_(x)Ga_(1-x)As (x=0.9) and p-Al_(x)Ga_(1-x)As (x=0.3) layers, thereis inserted a composition inclined layer (not shown), similarly to then-DBR mirror 304. A p-GaAs contact layer 314 is formed on the top formaking contact with the electrode. Between the n-DBR mirror 304 and thep-DBR mirror 312, the length amounts to 1 oscillation wavelength (thatis, the so-called random cavity is employed).

MOCVD is used for the thin-film forming (or crystal growing) method. H₂is used for a carrier gas, and trimethylgallium (TMG), trimethylaluminum(TMA) and arsine (AsH₃) are used as raw materials for the AlGaAs welllayer. H₂Se is used as an n-type dopant, and CBr₄ is used as a p-typedopant. The MOCVD can easily form the layers such as the compositioninclined layer, by controlling the amount of raw material gas that issupplied. Hence, the MOCVD is more suitable for producing the VCSELincluding the DBR mirror when compared to the MBE. In addition, theMOCVD is suited for mass production since it does not require a highvacuum as in the case of the MBE, and it is sufficient to control theflow rate and the supplying time of the raw material that is supplied.

An AlAs selectively oxidizing layer 316 is formed at a portion of thep-DBR mirror 312 having a low index of refraction and close to thequantum well active region 308. A mesa 318 having a predetermined sizeis formed in a state where the side surface of the p-AlAs selectivelyoxidizing layer 316 is exposed. Similarly as described above, the p-AlAsselectively oxidizing layer 316 having the exposed side surface isoxidized from the periphery, and an AlxOy current constricting part 320and a current injecting part 322 surrounded thereby are formed. Next,the etched portion is filled with polyimide 324 and planarized. Then,the p-contact layer 314 and the polyimide 324 existing above the p-DBRmirror 312 where a light emitting portion 326 is provided areeliminated, so as to form a p-electrode 328 on the p-contact layer 314at a portion other than the light emitting portion 326. In addition, ann-electrode 330 is formed on the back surface of the substrate 302 tothereby form the VCSEL having the oscillation wavelength of 780 nm.

In this embodiment, an AlAs layer is used as the selectively oxidizinglayer, but the selectively oxidizing layer may include other elementssuch as Ga. In addition, the content of the selectively oxidizing layermay be set larger than that of an AlGaAs layer forming the DBR mirror,so as to make the oxidation rate of the DBR mirror faster than that ofthe selectively oxidizing layer. In addition, the material used for thequantum well active region may be changed, and a semiconductor layerincluding Al and As may be selectively oxidized to form the currentconstricting structure of the VCSEL. Furthermore, the present inventionis not limited to the VCSEL, and is similarly applicable to other typesof lasers such as the edge-emitting laser. In other words, the presentinvention is similarly applicable to the production of semiconductorelements having a structure such that the current constricting structureis formed by selectively oxidizing a semiconductor layer including Aland As.

[First Application]

FIG. 16 is a plan view showing a VCSEL array chip using the VCSEL. InFIG. 16, a VCSEL array chip 400 has VCSELs 402 arranged in a line. EachVCSEL 402 is produced by any of the embodiments and modificationsdescribed above. The internal diameter of the current injecting partsurrounded by the current constricting part is 5 μm, and the VCSEL 402carries out the single mode operation in the lateral mode. In this firstapplication, the VCSEL 402 is formed on a p-type Gas semiconductorsubstrate, and an n-individual electrode 404 is provided on top while ap-common electrode (not shown) is provided on the bottom. Although theVCSELs 402 are arranged one-dimensionally in FIG. 16, the VCSELs 402 mayof course be arranged two-dimensionally.

According to the VCSEL array chip 400 having the VCSELs 402, thearrangement of the VCSELs 402 is simple because the VCSELs 402 aresurface-emitting types. In addition, the VCSELs 402 may be formedintegrally by a normal semiconductor process, and the VCSELs 402 may bearranged with an extremely high accuracy. For this reason, the yield ofthe VCSEL array chip 400 is greatly improved, and the production timecan be greatly reduced, to thereby enable inexpensive VCSEL array chips400 to be produced.

[Second Application]

FIG. 17 is a diagram showing an optical transmission module using theVCSEL. An optical transmission module 500 shown in FIG. 17 uses VCSELs.More particularly, the optical transmission module 500 includes theVCSEL array chip 502 shown in FIG. 16, and a plurality of opticalwaveguides 504. Each optical waveguide 504 confronts the end surface(light output part) of a corresponding VCSEL 402, so that the lightemitted from this corresponding VCSEL 402 is input to the opticalwaveguide 504. The optical waveguide 504 may be formed by an opticalfiber, for example. The optical fiber may be made of silica fiber, forexample. In this particular application, a single mode element whichemits light having a wavelength of 1.3 μm is used as the VCSEL 402, anda single mode fiber is used for the optical fiber forming the opticalwaveguide 504.

According to the optical transmission module 500 having the structuredescribed above, a high-speed parallel transmission is possible becausethe VCSELs 402 are used as the light emitting means (or light generatingmeans), and a large amount of data can be transmitted simultaneously. Inaddition, since the current injecting part and the current constrictingpart of each of the VCSELs 402 are formed to have uniform sizes, it iseasy to set a driving current of a driving circuit in the opticaltransmission module 500. Moreover, it is possible to realize inexpensiveoptical transmission modules 500 and optical communication systemshaving a high reliability.

Although the VCSEL and the optical fiber (VCSEL 402 and opticalwaveguide 504) correspond 1:1 in the optical transmission module 500shown in FIG. 17, a plurality of VCSELs having different oscillationwavelengths may be arranged one-dimensionally or two-dimensionally, anda wavelength multiplexed transmission may be made, to further increasethe transmission speed. By using a GaAs quantum well layer for theactive region of the VCSEL to make the oscillation wavelength of theVCSEL 850 nm, it becomes possible to combine the VCSELs with multi-modefibers.

[Third Application]

FIG. 18 is a diagram showing an optical communication module using theVCSEL. As shown in FIG. 18, an optical communication (transmission andreception) module 600 includes a VCSEL 602 that is produced by thesecond embodiment described above, a photodiode 604 for reception, andan acrylic plastic fiber 606. A multi-mode element with an oscillationwavelength of 780 nm is used for the VCSEL 602.

According to the optical communication module 600 having the structuredescribed above, it is possible to realize an inexpensive opticalcommunication system because the VCSEL 602 can be produced at a low costas described above, and the plastic fiber 606 is inexpensive. Inaddition, since the fiber diameter of the plastic fiber 606 isrelatively large, the coupling of the plastic fiber 606 with otherfibers is facilitated, to thereby facilitate the forming of the opticalcommunication system. Hence, the optical communication module 600 issuited for an in-house communication system for home use or office use,and also for communication systems within relatively small equipments.

In the optical transmission using the acrylic plastic fiber, there arestudies to use the VCSEL having the oscillation wavelength of 650 nm bytaking into consideration the absorption loss of the acrylic plasticfiber, but it has not been reduced to practice due to the poorhigh-temperature characteristics of the acrylic plastic fiber.Accordingly, the light emitting diode is presently used as the lightemitting source. However, the light emitting diode is not suited forhigh-speed modulation, and in order to realize a high-speed transmissionexceeding 1 Gbps, it is essential to develop a new semiconductor laser.But according to the VCSEL having the oscillation wavelength of 780 nmproduced according to the present invention, the active region gain islarge, a high output is obtainable and good high-temperaturecharacteristics are obtainable compared to the VCSEL having theoscillation wavelength of 650 nm. For this reason, the VCSEL producedaccording to the present invention generates less heat compared to theVCSEL having the oscillation wavelength of 650 nm, and an inexpensivesystem can be realized in that no cooling is required for the VCSEL.

In addition, the optical communication system using the opticalcommunication module 600 described above may not only be used for thecommunication between equipments such as computers in LANs or the likeusing optical fibers, but also used for short-distance communication, asan optical interconnection for data transmission between boards ofequipments, between LSIs of the board, and between elements within theLSI.

Recently, the processing capabilities of LSIs and the like have improvedconsiderably, and there are demands to improve the communication speedat the portions where such LSIs and the like are connected. By using theoptical communication module 600, however, it is possible to realize anultra-high-speed computer system by changing the conventional electricalconnections of the signals within the system to optical interconnectionsat portions such as between the boards of the computer system, betweenthe LSIs of the board, and between the elements within the LSI.

Moreover, a plurality of computer systems and the like may be connectedby the optical transmission module 500 or the optical communicationmodule 600 described above, so as to build an ultra-high-speed networksystem. Since the VCSEL has a considerably low power consumptioncompared to the edge-emitting semiconductor laser and the VCSEL caneasily be arranged two-dimensionally, it is suited for use in a paralleltransmission type optical communication system.

[Fourth Application]

FIG. 19 is a diagram showing a general structure of an image formingapparatus using the VCSEL. For example, an image forming apparatus 700shown in FIG. 19 is the so-called composite apparatus or multi-functionperipheral (MFP) integrally having the functions of anelectrophotography type copying apparatus, a printing apparatus, afacsimile apparatus and the like. The image forming apparatus 700includes a photoconductive body 702 that forms an electrostatic imagebearing body, and an exposure system 704 that exposes by light auniformly charged outer peripheral'surface of the photoconductive body702. The exposure system 704 includes a light source 706 for outputtinglight corresponding to the image that is to be formed, a polygonalmirror 708 for scanning the photoconductive body 702 by reflecting thelight output from the light source 706, and a lens system 710 forimaging the light reflected by the polygonal mirror 708 on thephotoconductive body 702. A VCSEL array 712 is used for the light source706.

FIG. 20 is a plan view on an enlarged scale showing the light source 706for exposure in the image forming apparatus 700 shown in FIG. 19. Asshown in FIG. 20(A), the VCSEL array 706 has VCSELs 712 that have theoscillation wavelength of 780 nm arranged two-dimensionally. Moreparticularly, 16 VCSELs 712 are arranged in a 4×4 arrangement, with 4VCSELs 712 arranged at a predetermined pitch (40 μm) in a horizontaldirection in FIG. 20(A) (main scanning direction or an axial directionof the photoconductive body 702) and 4 VCSELs 712 arranged at apredetermined pitch (40 μm) in a vertical direction (sub scanningdirection or a rotating direction of the photoconductive body 702).Between two mutually adjacent columns extending in the verticaldirection, the positions of the VCSELs 712 in one column is shifted by10 μm in the vertical direction with respect to the corresponding VCSELs712 in the adjacent column. Accordingly, as shown in FIG. 20(B), 16spots 714 are formed on the photoconductive body 702 at intervals ofessentially 10 μm in the sub scanning direction.

According to the image forming apparatus 700 having the structuredescribed above, it is possible to simultaneously irradiate theplurality of laser beams emitted from the plurality of two-dimensionallyarranged VCSELs 712 on the photoconductive body 702. Hence, compared toan image forming apparatus using a single laser oscillation lightsource, it is possible to form the image at an extremely high speed. Inaddition, since the VCSELs 712 are suited for two-dimensionalintegration, it is possible to more easily increase the number of laserbeams compared to the conventional edge-emitting semiconductor laserarray. In the particular case described above, the dots can be formed atintervals of approximately 10 μm in the sub scanning direction, whichcorresponds to 2400 dpi. Moreover, the write intervals in the mainscanning direction can easily be controlled by adjusting the lightemission timings of the VCSELs 712. Furthermore, since 16 dots can bewritten simultaneously using the VCSEL array 706 described above, anextremely high printing speed can be achieved. Of course, the number ofVCSELs 712 that are arranged in the vertical direction and thehorizontal direction may be increased to further increase the printingspeed. By adjusting the intervals or pitch of the VCSELs 712, theintervals of the spots 712 in the sub scanning direction can beadjusted, and it is possible to realize a high-definition printing at ahigh density of 400 dpi or higher. Since the current injecting parts ofthe VCSELs 712 that are produced according to the present invention areuniformly formed, it is possible to positively form spots 712 having thetarget diameter.

[Other Applications]

In the fourth application described above, the VCSELs 712 are used forthe light source 706 for image writing. However, the VCSEL 712 may alsobe used for a light source for recording and/or reproducing informationwith respect to recording media such as CDs. Since the VCSELs 712produced according to the present invention are inexpensive and havesmaller power consumption compared to edge-emitting semiconductorlasers, the VCSELs 712 are particularly suited for use in portableoptical pickup apparatuses which have strict power consumptionrequirements.

This application claims the benefit of a Japanese Patent Application No.2005-037805 filed Feb. 15, 2005, in the Japanese Patent Office, thedisclosure of which is hereby incorporated by reference.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

1. A semiconductor oxidation apparatus comprising: a sealable oxidation chamber defined by walls; a base provided within the oxidation chamber and configured to support a semiconductor sample; a supply part configured to supply water vapor into the oxidation chamber to oxidize a specific portion of the semiconductor sample; a monitoring window provided in one of the walls of the oxidation chamber and disposed at a position capable of confronting the semiconductor sample supported on the base; a monitoring part provided outside the oxidation chamber and capable of confronting the semiconductor sample supported on the base via the monitoring window; and an adjusting part configured to adjust a distance between the base and the monitoring part.
 2. The semiconductor oxidation apparatus as claimed in claim 1, wherein: the base includes a heating table that is provided with a heater, and a sample table that is arranged on the heating table and is configured to support the semiconductor sample; and the adjusting part includes a base moving mechanism that is configured to integrally move the heating table and the sample table.
 3. The semiconductor oxidation apparatus as claimed in claim 1, wherein: the base includes a heating table that is provided with a heater, and a sample table that is arranged on the heating table and is configured to support the semiconductor sample; and the adjusting part includes a base moving mechanism that is configured to move the sample table.
 4. The semiconductor oxidation apparatus as claimed in claim 1, wherein: the monitoring window is provided on one of the walls above the semiconductor sample supported on the base; and the adjusting part includes an elevator mechanism that is configured to move at least one of the base and the monitoring part upwards and downwards.
 5. The semiconductor oxidation apparatus as claimed in claim 1, wherein the adjusting part includes a moving mechanism that is configured to move the base between a monitoring position where the semiconductor supported on the base is adjacent to the monitoring window and a receded position where the semiconductor sample supported on the base is separated from the monitoring window by a longer distance that at the monitoring position.
 6. The semiconductor oxidation apparatus as claimed in any of claims 1 to 5, further comprising: an interrupting part configured to interrupt oxidation of the semiconductor sample by stopping the supply of the water vapor from the supply part; a part configured to obtain an oxidation rate of the specific portion of the semiconductor sample based on an image obtained by the monitoring part, and to obtain an amount of additional oxidation that is required based on the rate of oxidation; and a part configured to additionally oxidize the specific portion of the semiconductor sample by the amount of additional oxidation.
 7. The semiconductor oxidation apparatus as claimed in any of claims 1 to 6, wherein the monitoring part includes a microscope.
 8. The semiconductor oxidation apparatus as claimed in any of claims 1 to 7, wherein the monitoring part has an automatic focusing function.
 9. The semiconductor oxidation apparatus as claimed in any of claims 1 to 8, further comprising: a vacuum source configured to exhaust an atmosphere gas within the oxidation chamber by vacuum while interrupting oxidation of the semiconductor sample.
 10. The semiconductor oxidation apparatus as claimed in any of claims 1 to 8, further comprising: an inert gas supply part configured to spray or blast an inert gas onto the semiconductor sample supported on the base within the oxidation chamber while interrupting oxidation of the semiconductor sample.
 11. The semiconductor oxidation apparatus as claimed in any of claims 1 to 10, wherein the semiconductor sample is made up of a wafer for forming a vertical-cavity surface-emitting laser (VCSEL).
 12. A method of producing a semiconductor element by placing within a water vapor atmosphere a semiconductor sample that includes a mesa having a semiconductor layer including Al and As, and forming a current constricting part and a current injecting part that is surrounded by the current constricting part in the semiconductor layer by oxidizing the semiconductor layer from a peripheral end of the semiconductor layer appearing at an outer peripheral side surface of the mesa towards an inner radial direction so as to leave a central portion of the semiconductor layer non-oxidized, said method comprising the steps of: (a) interrupting an oxidation process at least once during oxidation of the semiconductor layer; and (b) monitoring an oxidation rate of the semiconductor layer while the oxidation process is interrupted.
 13. The method as claimed in claim 12, wherein the step (b) includes: (b1) moving the semiconductor sample within an oxidation chamber to a position where the mesa is adjacent to a monitoring part provided outside the oxidation chamber via a monitoring window of the oxidation chamber while the oxidation process is interrupted; and (b2) obtaining the oxidation rate based on a size of the current constricting part or the current injecting part that is monitored by the monitoring part.
 14. The method as claimed in claim 13, further comprising the steps of: (c) obtaining an amount of additional oxidation that is to be made based on the oxidation rate; and (d) additionally oxidizing the semiconductor layer by the amount of additional oxidation.
 15. The method as claimed in claim 13 or 14, wherein said step (b1) moves the semiconductor sample to a position where the mesa is adjacent to the monitoring window while the oxidation process is interrupted.
 16. The method as claimed in any of claims 13 to 15, further comprising the steps of: (e) exhausting an atmosphere gas within the oxidation chamber by vacuum while the oxidation process is interrupted.
 17. The method as claimed in any of claims 13 to 15, further comprising the steps of: (f) spraying or blasting an inert gas onto the semiconductor sample within the oxidation chamber while the oxidation process is interrupted.
 18. The method as claimed in any of claims 12 to 17, wherein the semiconductor element is a vertical-cavity surface-emitting laser (VCSEL). 